One goal in modern semiconductor fabrication is to improve the density of active elements provided on a single semiconductor die, thus increasing the number of die per wafer. As is known in the art, very large scale integration (VLSI) has evolved into ultra-high large scale integration (ULSI). In order to improve density without overly increasing die size, and more importantly, to improve device speed, there is ongoing investigation in decreasing further the critical dimension (CD) of active elements provided on the semiconductor die. Lithographic techniques are typically used in the formation of multilevel circuits on a semiconductor die. Currently, lithographic techniques take advantage of i-line (365 nanometer) and deep ultra-violet (DUV, 248 nanometer) energy sources. By decreasing wavelength of the energy utilized in these lithographic techniques, smaller CD's may be realized. Accordingly, smaller wavelength, higher energy sources have been investigated, including DUV (193 nanometers), EUV (extreme ultra-violet, approximately 11.0 to 13.4 nanometers), and X-ray lithography.
Another lithographic technique, projection electron beam lithography, shows potential in meeting the needs of several generations of semiconductor devices, including increased throughput and fine critical dimension (CD) control. In general, a projection electron-beam lithography system scans a beam across a mask to create an image on the semiconductor device. Electron optics can be inserted to provide a means of image reduction. One particular type of projection electron beam lithography is known as Scattering with Angular Limitation in Projection Electron-Beam Lithography. The basic principles of this technique are illustrated in prior art FIG. 1.
Turning to FIG. 1, the basic principles of SCALPEL are illustrated. As shown, a mask 10 having a patterned scattering layer 14 is provided on membrane 12, through which an electron beam is projected as represented by the arrows. Particularly, the patterned scattering layer has a higher atomic number than that of the membrane. The scattering effect of the electron beam through portions of the mask is illustrated in FIG. 1. As shown, those portions of the electron beam that pass through the scattering layer 14 tend to be scattered to a greater extent as compared with those portions that pass through the membrane material having no scattering layer 14.
As shown, the electron beam which passes through the mask, is focused through an electron focusing system, represented by lens 20. The electron beam then passes through back focal plane filter 30 having an aperture that is provided to permit passage of those portions of the electron beam that were not scattered by the scattering layer of the mask 10, through some finite angle. The electron beam is then projected onto a semiconductor wafer 40 having a plurality of die 42 and a resist layer 44 formed thereon by conventional techniques such as by spinning-on. The electron beam forms a high contrast image including areas of high intensity formed by those portions of the electron beam that pass between patterned portions of the mask 10, and areas of relatively low intensity formed by those portions of the electron beam that pass through the patterned areas of the mask 10. In this way, a high-resolution image may be projected onto the resist layer, which is then developed to form a patterned resist layer. Thereafter, the material exposed through the patterned resist layer may be etched using an appropriate etchant. It is noted that the power of the system may be adjusted so as to provide a 3-5.times. reduction in image size, typically 4.times..
Turning to FIGS. 2-1 to 2-4, a typical process for forming a mask for SCALPEL use is illustrated. First, a silicon substrate 102, such as on the order of 300-800 microns in thickness and 100-300 millimeters in diameter, is provided. The substrate 102 is formed of monocrystalline silicon, but other materials may be utilized. The substrate 102 is subjected to an LPCVD (low pressure chemical vapor deposition) process to form silicon-rich silicon nitride bottom layer 100 and membrane layer 104 on opposing major surfaces of the substrate. Layers 100 and 104 are typically on the order of 1,500 angstroms of thickness. Thereafter, an etch stop layer 106 is deposited upon membrane layer 104, typically on the order of 100 angstroms in thickness. A scattering layer 108 on the order of 300 angstroms in thickness is then provided on the etch stop layer 106. Typically, the etch stop layer is formed of Cr, while the scattering layer may be formed of any one of several high atomic number species, such as W, Ta, silicides and/or nitrides thereof. Preferably, at least one element has an atomic number greater than 72.
Turning to FIG. 2-2, an opening 103 is etched in the substrate 102, thereby leaving window portion 109 of relatively small thickness that spans opening 103. A resist 110 is coated and patterned on scattering layer 108, as shown in FIG. 2-3, and scattering layer 108 is etched so as to form patterned scattering layer 108', as shown in FIG. 2-4. According to the final structure shown in FIG. 2-4, the electron beam may pass through the entirety of the window portion 109, and is largely blocked by the substrate 102 along un-etched portions thereof. As shown, the window portion is composed of layers 104, 106, and 108' in FIG. 2-4.
Turning to FIG. 3, a plan view of an entire mask for SCALPEL is illustrated. The mask 10 includes substrate 102, preferably a silicon wafer. As illustrated, the mask 10 includes an array of window portions 109 that are covered by membrane layer 104, as well as by etch stop layer 106 and patterned scattering layer 108'. In the embodiment shown, a 4.times.34 array of windows is illustrated. However, it is well understood that the number of windows may be modified as understood by one of ordinary skill in the art. Clearly, should a larger substrate be utilized, a larger array may also be incorporated. It is noted that reference numeral 114 represents an alignment feature.
While SCALPEL technology has been demonstrated to provide improved resolution over conventional techniques, including i-line and DUV processing, the present inventors have recognized numerous deficiencies with conventional SCALPEL technology, particularly the mask utilized therefore illustrated hereinabove. More specifically, as stated above, the membrane that spans each of the openings is generally formed of silicon-rich silicon nitride. This particular material is conventionally used because of its stress properties. That is, it has a relatively low tensile stress, on the order of 2.times.10.sup.9 dynes/cm.sup.2, which is effective to maintain membrane integrity. For example, should the membrane have a compressive stress, the membrane would tend to wrinkle and not provide a proper surface for supporting a scattering layer. On the other hand, a membrane that has a high tensile stress has a tendency to fracture and can not be easily handled. While silicon-rich silicon nitride has adequate integrity as a membrane for supporting a scattering layer, the present inventors have discovered that following etching of the substrate, "pinholes" have been observed in membranes of various masks. Acordingly, there is a need in the art to provide an improved mask having low defectivity while maintaining high structural integrity for use in SCALPEL technology.